The present invention concerns the qualification of an electrooptical image sensor, the possible image defects of which may be masked by electronic processing in an image chain. The qualification is to determine whether the number and/or distribution of image defects of the sensor allows such masking or not without disturbing the proper operation of the image chain. The sensor can have a surface or a line of sensitive elements on which an image (optical, radiological or other) is presented or which in response produces signals in the form of pixels to reproduce that image, for example, on a monitor screen.
In the sensor it is almost impossible to obtain a correct operation of all the sensitive elements. In fact, the presence of defective elements is generally discovered after manufacture of the sensor, as is the appearance of new defective elements in the course of use. An element is considered defective if it produces a signal which does not vary, or not in the manner desired, depending on the exposure received. For example, a defective element may not produce any signal, or a constant signal regardless of the degree of exposure received. A defective element gives rise to what is called a bad pixel.
A bad pixel constitutes a loss of image information on the site of the corresponding defective element. If the image obtained by a sensor is visualized without masking the bad pixels present, the latter appear as so many black or white spots.
To avoid such a visual inconvenience, so-called interpolation techniques have been employed, aimed at masking the bad pixels on the image visualized. The masking is carried out by a digital processing of the bad pixels in an image processing system. The technique generally used is to substitute the erroneous digital value of the bad pixel with a value which is estimated from one or more correct neighboring pixels. For example, a value which is the mean value of the neighboring pixels situated to the right, to the left, directly above and directly below can be assigned to the defective pixel.
A bad pixel thus processed thereby renders a luminous intensity which does not clash with that of its neighbors, removing the visual inconvenience. The operation consisting of assigning an interpolated value to a bad pixel is generally designated by the term “bad pixel correction,” although it does not strictly involve restoring the functioning of the defective element.
To be able to correct bad pixels that way, it is first necessary to know their position in the image. For this purpose, a calibration stage is undertaken in which the sensor is subjected to known uniform exposures, and where the picked-up signal of each element is measured in order to locate those which produced an unacceptable signal. Following that procedure, a cartography of the bad pixels of the sensor in question is obtained, which indicates their position according to a system of coordinates, for example, in terms of line and column number, and the good pixels to be counted for interpolation.
This cartography is registered in the image processing system so that the latter can, on the one hand, identify each bad pixel on image output and, on the other, make the correction with the neighboring good pixels. That processing involves, for each bad pixel, establishing a code which fixes the method of calculation. The kind of code used depends, among other things, on the type of sensor, its dimensions and the interpolation algorithm.
The calculation time necessary for processing of bad pixels is not negligible compared to the time allowed for reading and other image processing. For a given system of correction of bad pixels, that processing time is not a simple function of the total number of bad pixels, for it also depends on their distribution on the sensitive surface.
Of course, in applications where only static images are processed, for example, in mammography, or of images which follow each other only at low frequency, an additional delay can be admitted in appearance of the image on the monitor due to the processing of bad pixels.
On the other hand, that delay must be strictly limited when dynamic images are produced, for example, in angiography, where typically 30 images (or frames) per second come out. In that case, the time which can be allotted to the correction of bad pixels or to other processing cannot exceed the period between two successive frames, at the risk of accumulating delays in the sequence of frames supplied on output and of disturbing operation of the image chain.
Hence, it is useful to be able to determine in advance whether a sensor is not going to require a time for correction of the bad pixels too long to permit its use with a given image processing system.